The US made M855 is painted with a green tip to distinguish it from M193, so that troops do not shoot M855 in their 1 turn every 12 inch twist barreled M16's that would fail to properly stabilize the heavier and much longer bullet (length being a primary factor in bullet stabilization).
Many, including myself believe that the 55gr M193 is a better strictly anti-personnel round than the 62gr M855, especially since it’s lighter weight allows it to fragment out to longer ranges. Also because the fragmentation of the M193 round is more reliable and more violent. When our NATO allies got on board with the 5.56 round, they had been unhappy with the overall penetration abilities of the 55gr M193 in a combat environment, specifically at longer ranges. They introduced the NATO SS109 62gr steel core “penetrator” bullet that would eventually be used in our M855 round.
A surplus British made Radway Green round, close to M855 spec, specifically because of the SS109 bullet. However, the powder is slightly different and seems to cause issues with some AR15's, as this round is designed to be used specifically in the British military's SA80 (also known as the L85). Works fine in my AR15's.
To the credit of the M855, it’s the better of the two rounds at longer ranges due to its size and ballistic coefficient. It’s not all that bad at shorter ranges either. The steel penetrator also makes it more effective at penetrating hard targets and such.
Keep in mind that 62gr M855 will not necessarily hold the same zero as other types of 62gr ammo. Because of the steel core (steel is lighter than lead), the SS109 bullet is actually longer than a completely lead core 62gr bullet would be. Additionally, the SS109 bullet has a small air cavity in the front of it that increases the length of the bullet even more.
You can find commercial offerings of these loadings. XM193 and XM855 are both produced by Federal (who operates the Lake City plant in Independence Missouri that produces ammo for the military). These are believed to be production overruns or in some cases "seconds", which may have been declared as such due to a small sampling not passing military specifications in one way or another. Despite the possibility that they may be seconds, the quality tends to be very good and I feel that it makes for good, hot, reliable (and fairly accurate) bulk "stash", home/personal defense ammo. It is not "surplus" ammo, as it is illegal for the military to sell surplus ammo to civilians. You can thank Bill Clinton for that (among other things).
Both types shoot just fine in all of my AR15's, with 1/9 or 1/7 twist rates.
Other similar offerings are found by Prvi Partizan and others (including NATO nations who sell surplus ammo). It tends to be waterproof (sealed) on both ends, crimped (to prevent bullet setback, especially in semi-autos), and annealed (keeps the neck from splitting by making it less brittle and softening the brass). It tends to be much cheaper than Federal XM193. I've shot several thousand rounds of it and not had one issue with it. So, it is my primary training and plinking ammo.
NATO ammo will have the cross and circle (+) headstamp on the bottom of the case. It is not definitive proof that the ammo is NATO spec though, as some manufacturers make ammo with that same brass (such as Federal American Eagle and even XM193) and some remanufacture ammo using once shot brass cases (like Black Hills Blue Box). Also realize that NATO spec and US military spec are not the exact same. The US military has certain specifications and standards that the ammo must meet.
Radway Green headstamp, NATO circle-cross (top), "RORG" means Royal Ordnance Radway Green and "93" is the year of manufacture (1993).
Not all 55-62gr bullets will fragment. Bullet construction varies and plays a huge role in whether or not the bullet will fragment at any given velocity.
Why Does The Military Use FMJ instead of JHP/SP's
Well, technically, they aren’t always limited to FMJ. Although they are forbidden "to employ arms, projectiles, or material calculated to cause unnecessary suffering."
Contrary to popular belief, this is according to the Hague Convention, not the Geneva Convention. We were not a party to either of these, but we tend to abide by the rulings on our own accord anyways.
The US can and does (not in general though) use JHP's. About 20 years ago, a US judge decided that it was perfectly legal to carry JHP's for special purposes.
"...expanding point ammunition is legally permissible in counter terrorist operations not involving the engagement of the armed forces of another state."
Luckily, the US military’s ammo selections have allowed them to cheat a little bit without really breaking the rules. For instance, the emphasis on the Mk262 77gr OTM was accuracy, not on bullet performance. The OTM (Open Tip Match) is not meant to increase bodily damage but increase in-flight stability and ballistic coefficient/speed. The excellent terminal performance is a bonus of the design though.
More on ammunition.